Cadmium induces apoptosis in a human T cell line

Oxidative stress and apoptosis in metal ion-induced carcinogenesis

Epidemiological evidence suggests that exposure to certain metals causes carcinogenesis. The mechanisms of metal-induced carcinogenesis have been pursued in chemical, biochemical, cellular, and animal models. Significant evidence has accumulated that oxidative stress may be a common pathway in cellular responses to exposure to different metals. For example, in the last few years evidence in support of a correlation between the generation of reactive oxygen species, DNA damage, tumor promotion, and arsenic exposure has strengthened. This article summarizes the current literature on metal-mediated oxidative stress, apoptosis, and their relation to metal-mediated carcinogenesis, concentrating on arsenic and chromium.

Cadmium-induced apoptosis through the mitochondrial pathway in rainbow trout hepatocytes: involvement of oxidative stress

Cadmium (Cd) induces oxidative stress and apoptosis in trout hepatocytes. We therefore investigated the involvement of the mitochondrial pathway in the initiation of apoptosis and the possible role of oxidative stress in that process. This study demonstrates that hepatocyte exposure to Cd (2, 5 and 10 microM) triggers significant caspase-3, but also caspase-8 and -9 activation in a dose-dependent manner. Western-blot analysis of hepatocyte mitochondrial and cytosolic fractions revealed that cytochrome c (Cyt c) was released in the cytosol in a dose-dependent manner, whereas the pro-apoptotic protein Bax was redistributed to mitochondria after 24 and 48 h exposure. We also found that the expression of anti-apoptotic protein Bcl-xL, known to be regulated under mild oxidative stress to protect cells from apoptosis, did not change after 3 and 6 h exposure to Cd, then increased after 24 and 48 h exposure to 10 microM Cd. In the second part of this work, two antioxidant agents, 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) (100 microM) and N-acetylcysteine (NAC, 100 microM) were used to determine the involvement of reactive oxygen species (ROS) in Cd-induced apoptosis. Simultaneously exposing trout hepatocytes to Cd and TEMPO or NAC significantly reduced caspase-3 activation after 48 h and had a suppressive effect on caspase-8 and -9 also, mostly after 24 h. Lastly, the presence of either one of these antioxidants in the treatment medium also attenuated Cd-induced Cyt c release in cytosol and the level of Bax in the mitochondria after 24 and 48 h, while high Bcl-xL expression was observed. Taken together, these data clearly evidenced the key role of mitochondria in the cascade of events leading to trout hepatocyte apoptosis in response to Cd and the relationship that exists between oxidative stress and cell death.

Proteome Analysis Associated with Cadmium Adaptation in U937 Cells

Cadmium is a well known environmental toxicant and carcinogen. To identify proteins involved in cellular adaptive responses to cadmium, we established cadmium-adapted U937 cells that exhibit resistance to cadmium-induced apoptosis, and we performed comparative proteome analysis of these cells with parental cells that were either untreated or treated with cadmium. Newly identified proteins that were changed in expression level in both adapted cells and cadmium-treated parental cells included proteins implicated in cell proliferation and malignant transformation. Most interesting, a calcium-binding protein calbindin-D(28k) was increased only in the adapted cells but not in cadmium-exposed parental cells. The level of calbindin-D(28k) increased by the degree of cadmium adaptation and was stably maintained without selective pressure of cadmium. Cadmium-adapted U937 cells were resistant to the toxic effects of cytosolic calcium rise by cadmium treatment and by depletion of intracellular calcium stores, suggesting that enhanced calcium buffering by up-regulated calbindin-D(28k) may be responsible for acquiring resistance to cadmium-induced apoptosis. We demonstrated that overexpression of calbindin-D(28k) in MN9D neuronal cells resulted in reduced cadmium-induced apoptosis. Our study documents for the first time that cells respond to long term cadmium exposure by increasing calbindin-D(28k) expression, thereby attenuating cadmium-induced apoptosis.

Whole-mount in situ TUNEL method revealed ectopic pattern of apoptosis in cadmium treated naupliar larvae of barnacle (Balanus amphitrite Darwin)

The effect of cadmium on stage II naupliar larvae of barnacle (Balanus amphitrite Darwin) was investigated. Barnacle larvae were exposed to 0-15 microM CdCl(2) for 24 h. Apoptotic cells were stained by the terminal deoxyribonucleotidyl transferase mediated dUTP nick end labelling (TUNEL) method. Incidence of apoptosis, as measured by numbers of animals with ectopic pattern of apoptosis as well as numbers of apoptotic cells per animal, was assessed using confocal microscopy. An increase in incidence of apoptosis was observed in the experimental animals with an increase in cadmium concentration. Mortality increased, and motility decreased, when barnacle larvae were exposed to an increasing concentration of cadmium. The relationship between the occurrence of apoptosis and swimming behaviour was investigated in larvae exposed to 10 microM CdCl(2). Significant increases in apoptosis were detected in the non-motile and dead nauplii. This study suggested that whole-mount in situ TUNEL method may be used to study increased occurrence of apoptotic cells in crustacean larvae.

Calcium and Its Role in the Nuclear Translocation and Activation of Cytosolic Phospholipase A2in Cells Rendered Sensitive to TNF-Induced Apoptosis by Cycloheximide

In these experiments, we investigated the role of calcium as a second messenger in the apoptotic activation of cytosolic phospholipase A(2) (cPLA(2)). As our model, we used a murine fibroblast cell line (C3HA) that was induced to undergo apoptosis by a combination of TNF and cycloheximide. Using fura 2 Ca(2+) imaging, we found strong evidence for an intracellular calcium response after 1 h of treatment, which correlated with the onset of phosphatidylserine externalization, but preceded effector procaspase processing by several hours. The response was strongest in the perinuclear region, where mean levels rose 83% (144 +/- 14 nM in untreated cells vs 264 +/- 39 nM in treated), while cells displaying morphological evidence of apoptosis had the highest levels of calcium (250-1000 nM). Verapamil blocked this response, indicating an extracellular source for the calcium. Fluorescence microscopy revealed a pattern of nuclear translocation of cPLA(2) during apoptosis, which was also blocked by verapamil, indicating an important role for calcium in this process. In addition, we found that verapamil prevented the release of [(3)H]arachidonic acid from C3HA cells induced to undergo apoptosis by the chemotherapeutic agents vinblastine, melphalan, and cis-platinum. Together, these data suggest that calcium is important for cPLA(2) activation by diverse apoptotic stimuli.

Molecular and cellular mechanisms of cadmium carcinogenesis

Cadmium is a heavy metal, which is widely used in industry, affecting human health through occupational and environmental exposure. In mammals, it exerts multiple toxic effects and has been classified as a human carcinogen by the International Agency for Research on Cancer. Cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities. Cd2+ does not catalyze Fenton-type reactions because it does not accept or donate electrons under physiological conditions, and it is only weakly genotoxic. Hence, indirect mechanisms are implicated in the carcinogenicity of cadmium. In this review multiple mechanisms are discussed, such as modulation of gene expression and signal transduction, interference with enzymes of the cellular antioxidant system and generation of reactive oxygen species (ROS), inhibition of DNA repair and DNA methylation, role in apoptosis and disruption of E-cadherin-mediated cell-cell adhesion. Cadmium affects both gene transcription and translation. The major mechanisms of gene induction by cadmium known so far are modulation of cellular signal transduction pathways by enhancement of protein phosphorylation and activation of transcription and translation factors. Cadmium interferes with antioxidant defense mechanisms and stimulates the production of reactive oxygen species, which may act as signaling molecules in the induction of gene expression and apoptosis. The inhibition of DNA repair processes by cadmium represents a mechanism by which cadmium enhances the genotoxicity of other agents and may contribute to the tumor initiation by this metal. The disruption of E-cadherin-mediated cell-cell adhesion by cadmium probably further stimulates the development of tumors. It becomes clear that there exist multiple mechanisms which contribute to the carcinogenicity of cadmium, although the relative weights of these contributions are difficult to estimate.

Hsp27, Hsp70, and metallothionein in MDCK and LLC-PK1 renal epithelial cells: effects of prolonged exposure to cadmium

Cadmium is a widely distributed industrial and environmental toxin. The principal target organ of chronic sublethal cadmium exposure is the kidney. In renal epithelial cells, acute high-dose cadmium exposure induces differential expression of proteins, including heat shock proteins. However, few studies have examined heat shock protein expression in cells after prolonged exposure to cadmium at sublethal concentrations. Here, we assayed total cell protein, neutral red uptake, cell death, and levels of metallothionein and heat shock proteins Hsp27 and inducible Hsp70 in cultures of MDCK and LLC-PK1 renal epithelial cells treated with cadmium for 3 days. Treatment with cadmium at concentrations equal to or greater than 10 microM (LLC-PK1) or 25 microM (MDCK) reduced measures of cell vitality and induced cell death. However, a concentration-dependent increase in Hsp27 was detected in both cell types treated with as little as 5 microM cadmium. Accumulation of Hsp70 was correlated only with cadmium treatment at concentrations also causing cell death. Metallothionein was maximally detected in cells treated with cadmium at concentrations that did not reduce cell vitality, and further increases were not detected at greater concentrations. These results reveal that heat shock proteins accumulate in renal epithelial cells during prolonged cadmium exposure, that cadmium induces differential expression of heat shock protein in epithelial cells, and that protein expression patterns in epithelial cells are specific to the cadmium concentration and degree of cellular injury. A potential role for Hsp27 in the cellular response to sublethal cadmium-induced injury is also implicated by our results.

Chromium(VI) down-regulates heavy metal-induced metallothionein gene transcription by modifying transactivation potential of the key transcription factor, metal-responsive transcription factor 1

The robust induction of metallothionein-I and II (MT-I and MT-II) genes by several heavy metals such as zinc and cadmium requires the specific transcription factor metal-responsive transcription factor 1 (MTF1). Chromium (VI), a major environmental carcinogen, not only failed to activate these genes but also inhibited their induction by Zn2+ or Cd2+. The heavy metal-induced expression of another MTF1 target gene, zinc transporter 1 (ZnT-1), was also down-regulated by Cr6+. By contrast, the expression of two MTF1-independent Cd2+-inducible genes, heme oxygenase 1 (HO-1) and HSP-70, was not sensitive to Cr6+. Cr6+ did not also affect the expression of housekeeping genes such as GAPDH or beta-actin. Stable cell lines overexpressing variable levels of MTF1, the key transactivator of the MT genes, demonstrated differential resistance toward the inhibitory effect of Cr6+, indicating MTF1 as a target of chromium toxicity. The basal and inducible binding of MTF1 to metal response elements was not affected by treatment of cells with Cr6+. Transient transfection studies showed that the ability of MTF1 to transactivate the MT-I promoter was significantly compromised by Cr6+. The fusion protein consisting of a Gal-4 DNA binding domain and one or more of the three transactivation domains of MTF1, namely the acidic domain, proline-rich domain, and serine-threonine rich domain, activated the GAL-4-driven luciferase gene to different degrees, but all were sensitive to Cr6+. MTF1 null cells were prone to apoptosis after exposure to Zn2+ or Cd2+ that was augmented in presence Cr6+, whereas the onset of apoptosis was significantly delayed in cells overexpressing MTF1.

Effects of low concentrations of cadmium on immunoglobulin E production by human B lymphocytes in vitro

Exposure to cadmium (Cd) can cause a variety of biological effects including alterations of immune responses in animals and humans. Both immunosuppression and immunoenhancement have been reported. The present study was aimed at investigating the consequences of exposure to Cd on the human immunoglobulin (Ig) E synthesis, using purified peripheral blood B lymphocytes and IL-4 and anti-human CD40 monoclonal antibody (a-CD40 mAb) as stimuli. Low concentrations of Cd (0.1-10 microM) markedly inhibited production of IgE in a concentration-dependent manner. IgG production, in contrast to IgE, showed a tendency towards being enhanced by Cd, although with a certain individual variability; IgM production was not affected. Cd failed to alter immediate surface expression of the activation markers CD69 and CD23 indicating that early activation events were not impaired. However, the portion of activated B cells was diminished by Cd after stimulation for more than 24 h, paralleled by a concomitant decrease in viability and a subsequent reduction in proliferation. These data suggest that the mechanism of Cd action on activated B cells involved pathways that interrupted an effectively initiated cell activation and induced a cytotoxic signal. Results from this study thus provide further evidence for and new information on the immunotoxic and immunomodulatory effects of Cd on human immune responses.

Morphological characterisation of BGM (Buffalo Green Monkey) cell line exposed to low doses of cadmium chloride

Morphological changes in the Buffalo Green Monkey (BGM) cell line after exposure to a subcytotoxic dose (0.062 mM, equivalent to EC(10)-effective concentration 10%) of cadmium chloride have been evaluated. Cells were exposed for 24 h and the effects observed at the ultrastructural level by transmission and scanning microscopy. Using transmission electron microscopy, the most notable findings in treated cells were the presence of intranuclear inclusion bodies and thin intracytoplasmic granules associated to myelin figures and the presence of apoptotic bodies. Other morphological alterations included cell vacuolisation and a reduced cytoplasm volume, condensation of the mitochondria and a decreased number of cytoplasmic organelles, except lysosomes and autophagic vacuoles, which increased in number. Scanning electron microscopy pointed to a cell with a disrupted perinuclear region and a decrease in the number of surface microvilli. We conclude that the BGM cell line may be considered an useful tool for toxicological studies involving cadmium.

Apoptosis and necrosis: two distinct events induced by cadmium in cortical neurons in culture

(1) Cadmium is an extremely toxic metal commonly found in industrial workplaces, a food contaminant and a major component of cigarette smoke. Cadmium can severely damage several organs, including the brain. In this work, we have studied both the cadmium toxicity on rat cortical neurons in culture and the possible protective effect of serum. (2) Our results indicate that: (1) cadmium is taken up by the neurons in a dose and serum dependent way; (2) cadmium, at concentrations from 1 micro M or 10 micro M (depending on the absence or the presence of serum) up to 100 micro M, decreases the metabolic capacity, which was evaluated by the XTT (tetrazolium salt) test; (3) cadmium induces apoptosis and LDH (lactate dehydrogenase) release in a dose dependent way; (4) in a serum-free medium, the cadmium-induced apoptosis is accompanied by caspase-3 activation; (5) both the caspase-3 activation and the cadmium-induced apoptosis are reversed by N-acethyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), a selective caspase-3 inhibitor, indicating that the caspase-3 pathway is involved in cadmium-induced apoptosis in cortical neurons; and (6) the cadmium concentrations which produce caspase-3 activation do not modify the intracellular ATP levels; however, higher cadmium concentrations lead to both intracellular ATP depletion and ATP release, but do not increase the caspase-3 activity, indicating that cadmium also produces cellular death by necrosis. (3) These results suggest that cadmium induces either apoptosis or necrosis in rat cortical neurons, depending on the cadmium concentration.

Potential role of p53 on metallothionein induction in human epithelial breast cancer cells

The expression and induction of metallothionein has been associated with protection against oxidative stress and apoptosis. This study examines the effect of tumour suppressor protein p53 on metallothionein expression following CdCl2 treatment in eight human epithelial breast cancer cell lines differing in p53 and oestrogen-receptor status. Cells were treated with 10 microM CdCl2 for 24 h and metallothionein protein levels were measured by cadmium binding assay. MCF7 cells which are p53-positive (p53+) and oestrogen-receptor-positive showed a large induction in metallothionein synthesis by 10.79+/-1.36-fold. Other breast cancer cell lines which are p53-negative (p53-) and oestrogen-receptor-negative or weakly oestrogen-receptor-positive showed a small induction ranging from 1.40+/-0.10 to 3.65+/-0.30-fold. RT-PCR analysis showed an induction of metallothionein mRNA in MCF7 cells by about 1.61+/-0.08-fold, while in HCC1806 cells (p53-, oestrogen-receptor-negative) by 1.11+/-0.13-fold, and in MDA-MB-231 (p53-, oestrogen-receptor-negative) by 1.25+/-0.06-fold. Metallothionein localisation was determined by immunohistochemical staining. Prior to metal treatment, metallothionein was localised mainly in the cytoplasm of MCF7 and MDA-MB-231 cells. After treatment with 10 microM CdCl2 for 24 h, MCF7 cells showed intense nuclear and cytoplasmic staining for metallothionein, while MDA-MB-231 cells showed staining in the cytoplasm with weak nuclear staining. Apoptosis induced by 10-40 microM CdCl2 at time points between 4 and 48 h was examined with TUNEL assay. In MCF7 cells, apoptosis increased with higher concentrations of CdCl2, it peaked at 6-8 h and appeared again at 48 h for all concentrations of CdCl2 tested. In MDA-MB-231 cells, apoptosis remained at low levels for 10-40 microM CdCl2 at all time points. Studies on cadmium uptake showed similar uptake and accumulation of cadmium at 8 and 24 h in all the cell lines. The data demonstrate that treatment of epithelial breast cancer cells with 10 microM CdCl2 for 24 h caused a greater induction of metallothionein protein and mRNA expression in p53+ and oestrogen-receptor-positive cells as compared to p53- and oestrogen-receptor-negative or weakly oestrogen-receptor-positive cells. This effect may be associated with the occurrence of apoptosis and suggests a role for p53 and oestrogen-receptor on the expression and induction of metallothionein in epithelial cells.

Evaluation of cadmium-induced transcriptome alterations by three color cDNA labeling microarray analysis on a T-cell line

Beside heavy metals, cadmium (Cd(2+)) is a ubiquitous toxic metal with a well established apoptotic and genotoxic effect, chronic exposure of which has been involved in a variety of pathological conditions. In the present study, we investigated by 1455 genes cDNA microarrays the toxic and apoptotic effect of Cd(2+), on the T-cell line CCRF-CEM, applying a three laser differential analysis, on the same microarray slide. The cells were cultured for 6 and 24 h in the absence (control) or presence of Cd(2+) (10 or 20 microM), RNAs were extracted and the produced cDNAs were labeled with rhodamine derivatives fluorescent dyes. A microarray slide was simultaneously hybridized by the labeled cDNAs and analyzed. We found that, in relation to control, treatment of the cells for 6 h with 10 and 20 microM Cd(2+), induces up-regulation in 20 and 34 genes, respectively. Treatment for 24 h with 10 and 20 microM Cd(2+) induces up-regulation in 22 and 84 genes, respectively. Twenty-eight genes were found down-regulated only after treatment for 24 h with Cd(2+) 10 microM. These data suggest that Cd(2+) produces a time- and dose-dependent molecular cascade, induces disturbances in different subcellular compartments, influencing thereafter the normal cellular functions, the differentiation process, the malignant transformation and the cell death.

Cadmium-induced DNA synthesis and cell proliferation in macrophages: the role of intracellular calcium and signal transduction mechanisms

Cd(2+) exposure increases the risk of cancer in humans and animals. In this report, we have studied the effect of Cd(2+) on signal transduction and Ca(2+) mobilization in murine macrophages. At micromolar concentrations, Cd(2+) significantly increased cell division as judged by [3H]thymidine uptake and cell counts. Cd(2+)-treated cells continued to proliferate even after more than 4 weeks in culture. Cd(2+) (1 microM) treatment induced a 1.5- to 2-fold increase in cytosolic free Ca(2+), [Ca(2+)](i), which was transitory and/or oscillatory. The sources of this Ca(2+) included both inositol 1,4,5-trisphosphate (IP(3))-sensitive and -insensitive stores. Macrophage treatment with 1-(6-((17beta-3-methoxyestra-1,2,5(10)-triene-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122), an inhibitor of phosphatidylinositol-specific phospholipase C (PLC), decreased Cd(2+)-induced formation of IP(3) in a concentration-dependent manner (K(d) about 2 microM). This caused a concomitant, partial decrease in the effect of Cd(2+) on [Ca(2+)](i). Cd(2+) itself crosses the macrophage membrane in part via L-type Ca(2+) channels, but it also interacts with a cell surface membrane protein(s) coupled to a pertussis toxin-sensitive G protein. Use of selective inhibitors of signal transduction and the quantitation of the levels of phosphorylated MAPK/ERK-activating kinase-1 (MEK1), extracellular signal-regulated kinase-1 (ERK1), and p38 mitogen-activated protein kinase (MAPK) suggests that the effects of Cd(2+) are mediated by the p21(ras)-dependent MAPK, but not the phosphoinositide 3 (PI 3)-kinase signalling pathway. The effect of activating these pathways includes increased availability of the transcription factor NFkappaB as well as activation of the early genes c-fos and c-myc.

Estimation of increased concentration of intracellular Cd(2+) by fluo-3 in rat thymocytes exposed to CdCl(2)

Cadmium, an environmental pollutant, has been reported to induce apoptosis in murine lymphocytes. To reveal the mechanism of cadmium-induced apoptosis, one of important questions is whether cadmium increases intracellular concentration of Ca(2+) ([Ca(2+)](i)), Cd(2+) ([Cd(2+)](i)) or both. It is difficult to detect the increase in [Ca(2+)](i) using Ca(2+)-chelator-based fluorescent Ca(2+) indicators in the presence of Cd(2+) because of their sensitivity to Cd(2+). Therefore, the study on membrane response such as Ca(2+)-dependent hyperpolarization gives a clue to reveal whether the [Ca(2+)](i) or [Cd(2+)](i) is increased. Cadmium at concentrations of 3 μM or more dose-dependently augmented fluo-3 fluorescence in rat thymocytes, presumably suggesting an increased [Ca(2+)](i). However, the membranes were not hyperpolarized although the cells possess Ca(2+)-dependent K(+) channels. One may argue that cadmium inhibits Ca(2+)-dependent K(+) channels so that cadmium fails to hyperpolarize the membranes. It is unlikely because the [Ca(2+)](i) increased by A23187, a calcium ionophore, elicited the hyperpolarization in the presence of Cd(2+). Furthermore, the profile of cytotoxicity induced by cadmium, examined by ethidium bromide and annexin V-FITC, was different from that induced by A23187. Taken together, it is concluded that the application of cadmium increases the [Cd(2+)](i) rather than the [Ca(2+)](i) in rat thymocytes, resulting in the induction of cytotoxicity.

Interference of CD95 expression on human lymphocytes

The study presents the exogenous influence of cadmium in comparison with zinc on the apoptosis of human lymphocytes by CD95 expression and its kinetic changes. The salts of both metals were used in final concentrations of 20 microM in cell cultures with whole blood. The duration of cultivation was 18 and 90 hours. The expression of surface antigens was evaluated by flow cytometry with monoclonal antibodies. In cultures of not stimulated cells we found in average 51.54% CD95 positive lymphocytes. The kinetic study of untreated cells showed elevation after 18 hours of cultivation and a very low expression after 90 hours. The CD95 expression on lymphocytes in cell culture with cadmium and zinc was lower after 18 hours of cultivation than in untreated cells. After 90 hours cultivation we found low levels of CD95 expression on cells treated with cadmium and a great individual variability in the number of positive cells upon the influence of zinc.

Cadmium induces apoptosis partly via caspase-9 activation in HL-60 cells

Cadmium (Cd), a potent immunotoxic metal, induces apoptosis both in vitro and in vivo. However, the mode of action remains unclear. We previously reported that Cd-induced apoptosis was partly dependent on mitochondria. In the present study, we investigated the involvement of caspase-9, which is the apex caspase in the mitochondoria-dependent apoptosis pathway, in Cd-induced apoptosis in human promyelocytic leukemia HL-60 cells. A specific inhibitor of caspase-9, Z-LEHD-FMK, partly inhibited DNA fragmentation induced by Cd treatment in HL-60 cells. Moreover, treatment of HL-60 cells with Cd resulted in the appearance of Cytochrome c (Cyt c), a potent activator of caspase-9, in the cytosol at 3 h, which closely paralleled the activation of caspase-9. Caspase-9 is an initiator caspase that is a potent activator of downstream effector caspases such as caspase-3. Caspase-3 activation was subsequent to the Cyt c release at 6 h. DNA fragmentation, an index of induction of apoptosis, also appeared 6 h after Cd treatment. The effects were more pronounced at 9 h after Cd addition. A broad-specificity inhibitor of caspases, Z-Asp-CH(2)-DCB, inhibited caspase-3 activation and DNA fragmentation induced by Cd in a dose-dependent fashion. The results suggest that Cd-induced apoptosis is partly caused by caspase-9 activation triggered by Cyt c.

Metallothionein overexpression in human trophoblastic cells protects against cadmium-induced apoptosis

Proper functioning of trophoblastic cells is essential for maintenance of the placenta and development of the embryo/fetus. Exposure of trophoblasts to toxic exogenous factors, such as cadmium (Cd), perturbs placental function and affects fetal outcome. Cellular responses to Cd exposure include induction of the metal-binding protein, metallothionein (MT), and initiation of apoptosis. To analyze the functional relationship between cellular MT levels and apoptosis in trophoblasts, we have examined the effects of DNA transfection-mediated alterations in MT levels on trophoblastic function and apoptosis, with and without Cd exposure, using the trophoblast-like JEG-3 human choriocarcinoma cell line. JEG-3 cells stably transfected with human MT-IIa cDNA expression constructs, in either sense or antisense orientation, were unchanged in human chorionic gonadotropin (hCG) production or expression of the apoptotic markers, bcl-2 and CPP-32. However, MT overexpression significantly prolonged the recovery time of intracellular Ca flux, whereas reduced basal MT increased the incidence of apoptosis as determined by morphology and terminal deoxynucleotidyl end labeling (TUNEL) staining. Upon Cd exposure, a dose-dependent decrease in hCG secretion was seen in all JEG-3 cultures, without any correlation to basal MT expression. Basal MT levels, however, significantly affected the extent of apoptosis, the incidence being inversely related to basal MT level. These results suggest that while MT does not ameliorate heavy-metal induced perturbation of some trophoblastic functions, its expression is critical for protection of these cells from Cd-induced apoptosis and could act to maintain placental integrity in cases of maternal Cd exposure.

Effects of heavy metal ions on resting and antigen-activated CD4(+) T cells

Heavy metal environmental pollutants increase susceptibility of affected individuals to bacterial and viral infections, but the mechanisms responsible for this effect are not known. We established cellular in vitro systems to identify molecular targets for the action of heavy metal ions. We used two model systems to determine the effects of heavy metal ions on antigen-induced T lymphocyte responses. The first system was representative of primary antigen responses and utilized CD4(+) primary T lymphocytes derived from DO.11.10 T cell receptor transgenic mice. The second system represented a memory T cell phenotype and utilized the CD4(+) T helper 1 clone, pGL2. We measured the effects of the four heavy metals cadmium, lead, mercury, and vanadium on cytokine and proliferation responses by purified CD4(+) T cell to antigenic stimulation. Cytokine responses were differentially affected by lead and vanadium at concentrations that did not affect T cell proliferation in response to antigen. We also determined whether the metal ions induced apoptotic cell death. Mercury induced apoptosis at concentrations as low as 0.5 microM, whereas cadmium required a concentration of 100 microM. Lead (maximal concentration tested was 200 microM) and vanadium (100 microM) did not induce apoptosis. The results suggested that the different heavy metal ions differentially affected antigen-stimulated responses in T helper cells. These in vitro systems can now be applied to test whether heavy metal ions alter antigen-induced T cell signal transduction pathways in CD4(+) T helper cells.

DNA damage by cadmium and arsenic salts assessed by the single cell gel electrophoresis assay

Human exposure to metals is frequent due to their ubiquity, wide use in industry, and environmental persistence. Direct and indirect genotoxic effects of cadmium (Cd) and arsenic (As) were reported. However, the mechanisms of induction of genetic damage are not well known. The aim of the present work was to evaluate the degree of damage induced by Cd and As salts in a human lung fibroblasts cell line using the single cell gel electrophoresis assay (SCGE). MRC-5 cells were treated with cadmium chloride (CdCl(2)), cadmium sulfate (CdSO(4)), sodium arsenite (NaAsO(2)) and cacodylic acid (C(2)H(7)AsO(2)). A significant dose-dependent increment in the extent of DNA migration as well as in the percentage of cells with tails was observed (P<0.001) after treatment with CdSO(4) and NaAsO(2). Treatment with CdCl(2) induced a relatively low level of DNA strand breaks in comparison with that induced by CdSO(4). The increase migration observed with the three compounds could be originated either by the direct induction of DNA lesions or by the inhibition of excision repair mechanisms. On the other hand, cells treated with C(2)H(7)AsO(2) showed a decrease in the migration length with the three doses employed (P<0.001). The decrease in the rate of DNA migration could be a consequence of the induction of DNA cross-links by organic arsenicals.Cd and As salts induced DNA damage in fibroblast cells, detected as DNA migration in the single cell gel (SCG) assay. The distribution of DNA migration among cells as a function of dose revealed that the majority of exposed cells showed more DNA damage than cells obtained from control cultures. The potential for human exposure to both metals has been increased over the years due to the increment in their use. For this reason, elucidation of carcinogenic mechanisms is very important.

Cadmium induces apoptosis and genotoxicity in rainbow trout hepatocytes through generation of reactive oxygene species

Cadmium poses a serious environmental threat in aquatic ecosystems but the mechanisms of its toxicity remain unclear. The purpose of this work was first to determine whether cadmium induced apoptosis in trout hepatocytes, second to determine whether or not reactive oxygen species (ROS) were involved in cadmium-induced apoptosis and genotoxicity. Hepatocytes exposed to increasing cadmium concentrations (in the range of 1-10 microM) showed a molecular hallmark of apoptosis which is the fragmentation of the nuclear DNA into oligonucleosomal-length fragments, resulting from an activation of endogenous endonucleases and recognized as a 'DNA ladder' on conventional agarose gel electrophoresis. Exposure of hepatocytes to cadmium led clearly to the DEVD-dependent protease activation, acting upstream from the endonucleases and considered as central mediators of apoptosis. DNA strand breaks in cadmium-treated trout hepatocytes was assessed using the comet assay, a rapid and sensitive single-cell gel electrophoresis technique used to detect DNA primary damage in individual cells. Simultaneous treatment of trout hepatocytes with cadmium and the nitroxide radical TEMPO used as a ROS scavenger, reduced significantly DNA fragmentation, DEVD-related protease activity and DNA strand breaks formation. These results lead to a working hypothesis that cadmium-induced apoptosis and DNA strand breaks in trout hepatocytes are partially triggered by the generation of ROS. Additional studies are required for proposing a mechanistic model of cadmium-induced apoptosis and genotoxicity in trout liver cells, in underlying the balance between DNA damage and cellular defence systems in fish.

Genotoxic ability of cadmium, chromium and nickel salts studied by kinetochore staining in the cytokinesis-blocked micronucleus assay

The aneugenic and clastogenic ability of cadmium chloride(II), cadmium sulfate(II), nickel chloride(II), nickel sulfate(II), chromium chloride(III) and potassium dichromate(IV) have been evaluated through kinetochore-stained micronucleus test. Traditional genotoxicity assays evaluate DNA damage, gene mutations and chromosome breakage. However, these tests are not adequate to detect aneugenic agents that do not act directly on DNA. Staining kinetochores in the cytokinesis-blocked micronucleus assay is a useful way to discriminate between clastogens and aneuploidogens and may allow a rapid identification of aneuploidy-inducing environmental compounds. Human diploid fibroblasts (MRC-5) were employed. All compounds increased micronuclei frequency in a statistically significant way. However, increases in kinetochore-positive micronuclei frequencies were higher than in kinetochore-negative ones. The present work demonstrates the genotoxic ability of the cadmium and chromium salts studied. Aneugenic as well as clastogenic ability could be observed with this assay. Nickel salts, as it was expected because of their known weak mutagenicity, showed lower genotoxic effects than the other metal salts studied. As the test employed only allows the detection of malsegregation, it is proposed that this mechanism is at least one of those by which the tested metal salts induced aneuploidy. On the other hand, visualization of kinetochores in all experiments suggests that the compounds studied did not act by damaging these structures.

Critical role of calcium overloading in cadmium-induced apoptosis in mouse thymocytes

Cadmium (Cd) is a well-known environmental carcinogen and immunotoxin. Currently the direct cytotoxic effects of Cd on thymocytes are largely unexplored. The main objective of the present study was to investigate the apoptogenic property of Cd and the mechanisms involved, using primary cultured mouse thymocytes as a model. Cd-induced apoptosis in thymocytes was studied by TdT-mediated dUTP nick end-labeling assay and DNA gel electrophoresis. The results showed that Cd was able to cause apoptosis in mouse thymocytes in a time- and dose-dependent manner. Moreover, Cd exposure led to a rapid and sustained intracellular calcium (Ca2+) elevation, followed by caspase-3 activation and PARP cleavage, all of which preceded the characteristic DNA fragmentation. BAPTA-AM, a specific intracellular Ca2+ chelator, abolished Cd-induced Ca2+ overloading and subsequently inhibited caspase-3 activation, PARP cleavage, and apoptosis. It is believed that intracellular Ca2+ elevation may trigger caspase-3 activation either through mitochondria or through activation of Ca2+-dependent protease in Cd-treated thymocytes. Results from this study thus provide new information for a better understanding of the immunotoxic and immunomodulatory effects of Cd.

Involvement of the extracellular signal-regulated protein kinase (ERK) pathway in the induction of apoptosis by cadmium chloride in CCRF-CEM cells

When CCRF-CEM cells were incubated with 5-40 microM CdCl(2,) apoptosis was observed most clearly at 10 microM. Prior to the development of apoptosis, mitogen-activated protein kinases (MAPKs), i.e. extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, were activated with different sensitivity to CdCl(2) exposure. ERK and p38 MAPK were phosphorylated with incubation of 1 microM CdCl(2,) but higher than 20 microM CdCl(2) was required for the clear phosphorylation of JNK. In the time-course study, ERK and p38 MAPK were phosphorylated earlier than JNK after CdCl(2) exposure. The in vitro activities of MAPKs also increased in response to CdCl(2) exposure. Pretreatment with an intracellular Ca(2+) chelator, 1, 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA/AM), suppressed almost completely CdCl(2)-induced phosphorylation of JNK and p38 MAPK, but not ERK phosphorylation, indicating that the activation of JNK and p38 MAPK depends on the intracellular Ca(2+) but that of ERK does not. On the other hand, treatment with a MAPK/ERK kinase (MEK) inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene ), suppressed CdCl(2)-induced ERK activation and the apoptosis as well. The inhibition of p38 MAPK activity with SB203580 (4-[4-fluorophenyl]-2-[4-methylsulfinylphenyl]-5-[4-pyridyl]1H- imidaz ole) did not protect cells from apoptosis. The present results showed that the activation of ERK, JNK, and p38 MAPK is differently regulated in CCRF-CEM cells exposed to CdCl(2,) and that the ERK pathway seems to be responsible for the induction of apoptosis by CdCl(2) exposure in this human T cell line.

Apoptosis induced by cadmium in human lymphoma U937 cells through Ca2+-calpain and caspase-mitochondria- dependent pathways

Apoptosis induced by cadmium has been shown in many tissues in vivo and in cultured cells in vitro. However, its molecular mechanism is not fully understood. When the human histiocytic lymphoma cell line U937 was treated with cadmium for 12 h, evidence of apoptotic features, including change in nuclear morphology, DNA fragmentation, formation of DNA ladder in agarose gel electrophoresis, and phosphatidylserine externalization, were obtained. Moreover, loss of the mitochondrial membrane potential (Deltapsi(m)) was observed in the cadmium-treated cells and was inhibited by a broad caspase inhibitor (Z-VAD-FMK). Caspase inhibitors suppressed the DNA fragmentation in the order of Z-VAD-FMK > caspase-8 inhibitor > caspase-3 inhibitor. Expression of Bcl-x(L) and Bid decreased significantly in the cadmium-treated cells, although no apparent change in Bcl-2 and Bax expression was found. Tetrakis-(2-pyridylmethyl) ethylendiamine, a cell-permeable heavy metal chelator, partially reversed the increase of fluorescence of Fura-2 in the cadmium-treated cells. In addition, verapamil (70 microm), a voltage-dependent Ca(2+) channel blocker, inhibited the DNA fragmentation induced by cadmium less than 100 microm and decreased the fluorescence of Fura-2. Cadmium up-regulated the expression of type 1 inositol 1,4,5-trisphosphate receptor (IP(3)R) but not type 2 or type 3 IP(3)R. Calpain inhibitors I and II partially prevented DNA fragmentation. No effects of Z-VAD-FMK on the expression of type 1 IP(3)R or of calpain inhibitors on the loss of Deltapsi(m) were observed. These results suggest that cadmium possibly induced apoptosis in U937 cells through two independent pathways, the Ca(2+)-calpain-dependent pathway and the caspase-mitochondria-dependent pathway.

Activity and isoenzyme profile of N-acetyl-beta-D-glucosaminidase in urine from workers exposed to cadmium

The urinary excretion of N-acetyl-beta-D-glucosaminidase (U-NAG) and urinary Cadmium (U-Cd) concentration, a measure of the metal load in the body, were evaluated in 28 workers exposed to Cd, to determine the relation between the two parameters. In urine from 22 exposed workers with U-Cd<2 microg/g creatinine (Cr) there was no significant difference in U-NAG value (0.98+/-0.59 U/gCr) compared to non-exposed (0.73+/-0.48 U/gCr). In the six workers with 2 microg/gCr < or =U-Cd<10 microg/gCr the U-NAG (2.32+/-0.61 U/gCr) was statistically (P<0.05) higher than in other workers. In both the U-Cd intervals examined there were no altered values of beta2-microglobulin from urine of exposed workers compared to non-exposed (<0.30 mg/l). The U-NAG isoenzymes were separated by DEAE-cellulose chromatography from urine of non-exposed subjects and exposed workers. The U-NAG isoenzyme profile in urine of non-exposed subjects showed a high percentage (about 95%) of the A (acid) form, a much lower percentage (about 4.5%) of B (basic) form and a negligible percentage (about 0.5%) of I (intermediate) form. In the urine of 22 exposed workers with U-Cd<2 microg/gCr, the percentages of U-NAG isoenzymes were not different from non-exposed. In the urine of six workers with 2 microg/gCr< or =U-Cd<10 microg/gCr the percentage (8.34+/-0.91) of isoenzyme B (U-NAG-B), a marker of lesional enzymuria, was statistically increased (P<0.05) compared to non-exposed (4.42+/-0.56). These results suggest that adopting a biological limit for U-Cd equal to 10 microg/gCr might not be sufficiently protective. The increase in U-NAG-B content at 2 microg/gCr<U-Cd</=10 microg/gCr is probably due to a lesion of the proximal tubule caused by the metal which might follow an induction of the apoptotic process.

Acute cadmium exposure inactivates thioltransferase (Glutaredoxin), inhibits intracellular reduction of protein-glutathionyl-mixed disulfides, and initiates apoptosis

Oxidative stress broadly impacts cells, initiating regulatory pathways as well as apoptosis and necrosis. A key molecular event is protein S-glutathionylation, and thioltransferase (glutaredoxin) is a specific and efficient catalyst of protein-SSG reduction. In this study 30-min exposure of H9 and Jurkat cells to cadmium inhibited intracellular protein-SSG reduction, and this correlated with inhibition of the thioltransferase system, consistent with thioltransferase being the primary intracellular catalyst of deglutathionylation. The thioredoxin system contributed very little to total deglutathionylase activity. Thioltransferase and GSSG reductase in situ displayed similar dose-response curves (50% inhibition near 10 micrometer cadmium in extracellular buffer). Acute cadmium exposure also initiated apoptosis, with H9 cells being more sensitive than Jurkat. Moreover, transfection with antisense thioltransferase cDNA was incompatible with cell survival. Collectively, these data suggest that thioltransferase has a vital role in sulfhydryl homeostasis and cell survival. In separate experiments, cadmium inhibited the isolated component enzymes of the thioltransferase and thioredoxin systems, consistent with the vicinal dithiol nature of their active sites: thioltransferase (IC(50) approximately 1 micrometer), GSSG reductase (IC(50) approximately 1 micrometer), thioredoxin (IC(50) approximately 8 micrometer), thioredoxin reductase (IC(50) approximately 0.2 micrometer). Disruption of the vicinal dithiol on thioltransferase (via oxidation to C22-SS-C25; or C25S mutation) protected against cadmium, consistent with a dithiol chelation mechanism of inactivation.

Induction of apoptosis in mouse thymocytes by cadmium

In the thymus apoptosis is an important process in T cell maturation and differentiation. Cadmium (Cd) is an ubiquitous toxic metal that is capable of modulating immune responses. To investigate the induction of apoptosis and immunomodulation by environmental chemicals, we cultured mouse thymocytes with Cd and/or dexamethasone (DEX). DNA fragmentation was analyzed by gel electrophoresis, ELISA and flow cytometry. Treatment with either Cd or DEX induced DNA fragmentation in the thymocytes. Exposure to 10 microM Cd killed thymocytes by apoptosis rather than necrosis. However, no synergistic or additive effect was observed in the induction of apoptosis when DEX was added to the Cd. These results suggest that Cd may modulate the function of the thymus by the induction of apoptosis through mechanisms that differ from those used by DEX.

Evidence that E-cadherin may be a target for cadmium toxicity in epithelial cells

E-cadherin is a Ca(2+)-dependent cell adhesion molecule that plays an important role in the development and maintenance of epithelial polarity and barrier function. This commentary describes the results of recent studies showing that the environmental pollutant Cd(2+) can damage the E-cadherin-dependent junctions between many types of epithelial cells and reviews the evidence indicating that this effect results from the direct interaction of Cd(2+) with the E-cadherin molecule. In addition, the implications of these findings with respect to the mechanisms of Cd(2+) toxicity in specific target organs such as lung, kidney, bone, and the vascular endothelium are discussed.

Cadmium induces caspase-mediated cell death: suppression by Bcl-2

Apoptosis is a process of active cell death and is characterized by activation of caspases, DNA fragmentation, and biochemical and morphological changes. To better understand apoptosis, we have characterized the dose- and time-dependent toxic effects of cadmium in Rat-1 fibroblasts. Staining of cells with phosphatidylserine (PS)-annexin V, Hoechst 33258 or Rhodamine 123 and Tunel assays showed that incubating cells with 10 microM cadmium induced a form of cell death exhibiting typical characteristics of apoptosis, including cell shrinkage, externalization of PS, loss of mitochondria membrane potential, nuclear condensation and DNA fragmentation. Expression of Bcl-2 or CrmA each suppressed cadmium-induced cell death although Bcl-2 was somewhat more effective than CrmA. In vitro assay of caspase activity carried out using poly(ADP-ribose) polymerase (PARP) as a substrate as well as intracellular caspase assays using a fluorigenic caspase-3 substrate confirmed that caspase-3 is activated in Rat-1 cells undergoing cadmium-induced apoptosis. Both Asp-Glu-Val-Asp-aldehyde (DEVD-cho) and Tyr-Val-Ala-Asp-chloromethylketone (YVAD-cmk), selective inhibitors of caspase-3 and caspase-1, respectively, suppressed significantly cadmium-induced cell death. However, the nonselective caspase inhibitor, z-Val-Ala-Asp-floromethylketone (zVAD-fmk), was the most efficacious agent, almost completely blocking cadmium-induced cell death. Taken together, these results demonstrate that as in other forms of apoptosis, caspases play a central role in cadmium-induced cell death.

Activation of parallel mitogen-activated protein kinase cascades and induction of c-fos by cadmium

Cadmium is a toxic divalent cation that can initiate either mitogenic signals or apoptosis, possibly as a consequence of inducing different patterns of oncogene expression in different cells. We previously showed that Cd(2+) caused transcriptional activation of the c-fos protooncogene in mesangial cells (Wang and Templeton, J. Biol. Chem. 273, 73-79, 1998). The present study was undertaken to identify the signaling pathways that might be involved. Exposure to 10 microM CdCl(2) for 8 h caused a prolonged activation of Erk kinase and accumulation of c-fos mRNA. Inhibition of Erk activation with PD98059 only partially inhibited c-fos induction, indicating that additional pathways are involved. The c-Jun kinase/stress-activated protein kinase (SAPK) was also activated by Cd(2+). All three signals, i.e., Erk activity, SAPK activity, and c-fos mRNA levels in response to Cd(2+) showed a similar biphasic time course with an initial increase at 15-30 min and then a larger and more prolonged increase several hours later. Each signal also showed a similar concentration dependence, with less than 1 microM Cd(2+) causing the initial increase but values above 3 microM being required for the prolonged phase. These events showed high specificity for Cd(2+); other divalent metals tested under the same conditions (Mg(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), and Hg(2+)) were without significant effects. We conclude that Cd(2+) is a specific inducer of c-fos in mesangial cells, probably through activation of both Erk kinase and SAPK pathways. The similar time and concentration dependence of the response of both pathways to Cd(2+) suggests a common basis for activation.

Uncoupling of apoptosis and Jun/AP-1 activity in human promonocytic cells treated with DNA-damaging and stress-inducing agents

Earlier studies have indicated that Jun/AP-1 activity is associated with, and probably required for apoptosis induction by DNA-damaging and stress-inducing agents in human myeloid cells. To investigate this possibility, we examined the capacity of continuous treatments with etoposide (10 microM) and camptothecin (0.4 microM), and pulse treatments with X-rays (20 Gy), heat (2 h at 42.5 C) and cadmium chloride (2 h at 200 microM) followed by recovery, to provoke apoptosis and to simulate c-jun and c-fos expression and AP-1 binding in U-937 human promonocytic cells. All these treatments generated apoptosis with similar efficacy (50-60% apoptotic cells at 6 h of treatment or recovery). However, the capacity to increase c-jun and c-fos mRNA levels and to stimulate AP-1 binding was very different, ranging from more than a twelve-fold increase in the case of cadmium, to almost no increase in the case of heat-shock and etoposide. When the cells were pre-conditioned with a soft heat shock (1 h at 42 degrees C) the cadmium-provoked apoptosis was greatly inhibited, but the stimulation of AP-1 binding was not affected. The administration of cAMP-increasing agents also reduced the etoposide- and cadmium-provoked apoptosis. However, cAMP greatly stimulated c-jun and c-fos expression and AP-1 binding when applied together with etoposide (which itself was ineffective), and potentiated the cadmium-induced AP-1 binding. Conversely, retinoic acid abrogated the cadmium-provoked stimulation of AP-1 binding and transactivation capacity, and greatly inhibited the stimulation of binding caused by camptothecin and X-rays. However, retinoic acid did not inhibit the induction of apoptosis by these agents. These results indicate that Jun/AP-1 activity is not necessarily coupled with apoptosis, nor required for apoptosis induction by DNA-damaging and stress-inducing agents in human promonocytic cells.

Calcium channel blockers, apoptosis and cancer: is there a biologic relationship?

Calcium channel blockers (CCBs) represent a chemically and pharmacologically diverse group of agents that are widely used for the treatment of hypertension and angina. A small number of retrospective, observational analyses have raised concern about a potential causal link between CCB use and an increased risk for cancer development. Despite the absence of cancer findings in extensive preclinical studies, it has been proposed that CCBs may work differently in humans by interfering with apoptosis, leading to an increased potential for abnormal cell proliferation and tumor growth. This biologic hypothesis has attracted considerable attention in the medical community but has not been critically evaluated. An analysis of the basic and clinical literature was conducted to examine biologic relationships among cell Ca2+ modulation, apoptosis, and cancer. In addition to a comprehensive review of the cellular and animal data, the results of large observational studies were included in this analysis. Results of this review demonstrated that the effects of CCBs on apoptosis are complex as both increases and decreases in intracellular Ca2+ have been linked to this form of programmed cell death. Most studies show that an effect (either positive or negative) of CCBs on apoptosis requires doses in the supra-pharmacologic range, and are therefore not clinically relevant. Results of large and methodologically robust observational studies fail to provide support for the hypothesis that CCB use is associated with an increased susceptibility for cancer incidence. A comprehensive analysis of the basic and clinical evidence does not support a causal relationship between the therapeutic use of CCBs and an increased incidence of cancer development as a result of interfering with apoptosis.

Chronic cadmium toxicity to sperm of heavy cigarette smokers: immunomodulation by zinc

The aim of the study was to investigate the role of zinc therapy in 125 male cigarette smokers with infertility. The mechanism involved in the zinc/cadmium relationship was evaluated through the effect of a zinc-deficient diet and supplementation on testes of male adult Sprague-Drew rats. Heavy smoking was associated with low sperm count, motility, and morphology and increased seminal cadmium levels. Zinc therapy improved sperm quality and increased seminal IL-4, but reduced TNF-alpha and IFN-gamma. A zinc-deficient diet led to high cadmium testicular accumulation comparable with those supplemented with cadmium. Cadmium had a linear correlation with TNF-alpha and IFN-gamma, but not with IL-4. Cytology of testicular aspirate and histopathology were normal in supplemented groups as in controls. These results indicate that zinc modulates the putative effect of cadmium through its enhancement of T-helper 2 cytokines expression and down-regulation of T-helper 1 cytokines.

Oxygen stress: a regulator of apoptosis in yeast

Oxygen radicals are important components of metazoan apoptosis. We have found that apoptosis can be induced in the yeast Saccharomyces cerevisiae by depletion of glutathione or by low external doses of H2O2. Cycloheximide prevents apoptotic death revealing active participation of the cell. Yeast can also be triggered into apoptosis by a mutation in CDC48 or by expression of mammalian bax. In both cases, we show oxygen radicals to accumulate in the cell, whereas radical depletion or hypoxia prevents apoptosis. These results suggest that the generation of oxygen radicals is a key event in the ancestral apoptotic pathway and offer an explanation for the mechanism of bax-induced apoptosis in the absence of any established apoptotic gene in yeast.

Effects of calcium channel blockers on cellular apoptosis: implications for carcinogenic potential

BACKGROUND

A small number of well-publicized, retrospective epidemiologic reports have suggested a causal relation between the use of calcium channel blockers (CCBs) for the treatment of hypertension and an increased risk for cancer. The biologic mechanism proposed to explain this possible relation is that CCBs interfere with apoptosis, an active cell death process required for the regulation of normal cell populations in the body. Because an elevation in cellular calcium (Ca2+) is thought to be involved in apoptosis, it has been argued that CCBs could inhibit apoptosis, leading directly to tumor promotion.

METHODS

A comprehensive and critical review of the scientific literature was conducted specifically to evaluate the effects of pharmacologic CCBs on apoptosis and tumor development in various experimental models.

RESULTS

A review of the scientific literature revealed that CCBs have complex and often contradictory effects on cellular apoptosis. In various experimental models of cancer, CCBs did not promote neoplastic growth. By contrast, CCB treatment was associated with an inhibition of tumor growth in certain models of neoplasia and was also an effective adjuvant therapy in the treatment of certain drug-resistant tumors. Additional large epidemiologic studies have failed to support the hypothesis that CCB use is associated with an increased susceptibility for cancer.

CONCLUSIONS

A biologic link between the use of CCBs and increased human risk for cancer development as a result of modulating cellular apoptosis is not supported by the scientific literature.

Cadmium-bound metallothionein induces apoptosis in rat kidneys, but not in cultured kidney LLC-PK1 cells

The ability of cadmium-bound metallothionein(Cd-MT) to induce apoptosis was investigated in vivo and in vitro. Administration of purified Cd-MT (0.15 mg MT bound Cd per kg body weight) to the rat induces DNA fragmentation, a biochemical characteristic of apoptosis in the kidney at 16 h, which was detectable by ethidium bromide staining on an agarose gel. It was still detected 24 h after administration. Induction of apoptosis by Cd-MT was specific to kidney; it was not observed in cerebrum, cerebellum, heart, lung, liver, testis, dorsolateral prostate, and ventral prostate. In contrast, addition of Cd-MT (0.01-100 microM) to the cultured porcine kidney LLC-PK1 cells failed to induce apoptosis under the condition where cadmium chloride (10 microM) did. There was no additivity of induction of apoptosis by CdCl2 (10 microM) in the presence of Cd-MT (0.01-100 microM). To examine the effect of intracellular MT on cadmium-induced apoptosis in cultured cells, new cell lines were established, which constitutively produce MT, being termed as Cd(r)-LLC-PK1 cells since Cd-MT exogenously added had much less permeability to the cultured cells. Followed by exposure of wild-type LLC-PK1 cells to 50 microM CdCl2 for 24 h, the surviving cells(Cd(r)-LLC-PK1 cells) induce MT at the level of 1.9 microg/2 x 10(6) cells. In Cd(r)-LLC-PK1 cells, 10 microM CdCl2 failed to induce apoptosis, but 60 microM CdCl2 could exert the apoptotic response, indicating that intracellular MT which was induced by CdCl2 did not facilitate CdCl2-elicited apoptosis. Furthermore, chromatin in rat kidneys was condensed by Cd-MT, but not that in LLC-PK1 cells. Thus, Cd-MT induces apoptosis in rat kidneys, but not in the cultured renal cells, suggesting that the ionic form of cadmium was required for programmed cell death.

Characterization of cadmium-induced apoptosis in rat lung epithelial cells: evidence for the participation of oxidant stress

The mode of cadmium-induced cell death was investigated in a rat lung epithelial cell line. Cells, grown to near confluence, were exposed to 0-30 microM CdCl2 for 0-72 h. Phase contrast microscopy and fluorescent nuclear staining showed that Cd caused morphological alterations in lung epithelial cells that are characteristic of apoptosis. These changes included cell shrinkage, detachment of the cell from its neighbors, cytoplasmic and chromatin condensation, and fragmentation of the nucleus into multiple chromatin bodies surrounded by remnants of the nuclear envelope. Apoptotic DNA degradation was validated and quantitated using a sensitive enzyme-linked immunosorbent assay (ELISA) which measures the amount of histone-bound DNA fragments in the cytosol. Using this technique, a maximum level of apoptosis (5-fold higher than control) was observed in cultures exposed for 48 h to 20 microM CdCl2. The terminal deoxyribonucleotidyl transferase mediated dUTP nick end labeling method (TUNEL) was subsequently used to determine the percentage of cells that contained Cd-induced DNA strand breaks. After 48 h, approximately 54% of the cells exposed to 20 microM Cd were TUNEL positive compared to less than 2% for control cells. Although the mechanisms by which Cd initiates apoptosis in these cells are presently not known, reactive oxygen species are likely to play a role. This possibility is supported by the finding that the first morphological features indicative of apoptosis were preceded by the up-regulation of oxidant stress genes (glutathione S-transferase-alpha, gamma-glutamylcysteine synthetase, and metallothionein-1), activation of redox sensitive transcription factors (AP-1 and NF-kappaB), and changes in various forms of glutathione (reduced, oxidized, and protein-bound).

Distribution of cadmium and metallothionein in CdCl2-exposed rat kidney: relationship with apoptosis and regeneration

Male Sprague-Dawley rats were injected subcutaneously with 0.6 mg cadmium/kg bodyweight per day for 6 weeks. In each week of exposure, rats were killed and the localization of cadmium and metallothionein in the kidney was studied by histochemical and immunohistochemical methods. Although cadmium was localized throughout the proximal tubules during exposure, apoptosis and subsequent regeneration were observed mainly in the straight portion of the proximal tubules after 4 weeks of exposure. The distribution of tubular injury may thus not necessarily coincide with that of cadmium. Expression of metallothionein was also detected in the cytoplasm and nuclei of the convoluted and the straight portion of the proximal tubules, but the latter became positive in accordance with apoptosis and regeneration. These results suggest a close relationship between metallothionein distribution and tubular cell regeneration.

Apoptosis in dab (Limanda limanda) as possible new biomarker for anthropogenic stress

Apoptosis, also known as programmed cell death, is a physiological and irreversible process in tissue homeostasis that leads to DNA fragmentation of multiples of 180-200 bp. Because apoptosis can be initiated not only by physiological stimuli but also by various chemical substances, the present paper investigates the suitability of apoptosis as a biomarker for biological effect monitoring in the marine environment. Aquarium experiments with dab (Limanda limanda) were carried out to examine the effects of exposure to cadmium, PCB 118, and PCB 77 (each 1 mg/kg fish wt) on apoptosis in dab liver. Determination of apoptosis was carried out by DNA gel electrophoresis and quantification of DNA fragments smaller than 1500 bp. In addition, accumulated amounts of cadmium, PCB 118, and PCB 77 in dab liver were analyzed. Quantification of the three xenobiotics resulted in an accumulation of about factor 10(2)-10(4). Exposure to PCB 118 and cadmium resulted in an increase in apoptotic DNA fragmentation. Exposure to PCB 77 led mainly to cell death by necrosis.

Cadmium-induced inhibition of the growth and metastasis of human lung carcinoma xenografts: role of apoptosis

Our previous studies indicate that cadmium in mice can inhibit the formation of chemically induced and spontaneously occurring tumors in the liver and lung. Cadmium is an effective anti-tumor agent when given at non-toxic doses and even when given well after tumor formation, implying a unique sensitivity in certain tumor cells. The present studies tested the ability of cadmium to inhibit growth and progression of transplanted human pulmonary tumor xenografts. Male athymic nude mice were inoculated with either H460 cells, originally derived from a non-small cell pulmonary carcinoma, or DMS 114 cells, originally derived from a small cell lung carcinoma, under the left renal capsule. Starting 1 week later mice received 0, 125 or 250 p.p.m. cadmium in the drinking water, levels without effect on host animal growth or survival, and were observed over the next 4 weeks (H460 cells) or 100 days (DMS 114 cells). An additional experiment gave cadmium as an i.v. loading dose (20 micromol/kg) 4 days after renal inoculation with H460 cells and 200 p.p.m. cadmium in the drinking water from 7 days onward, with an observation period of 28 days. Cadmium caused dose-related reductions in the growth of tumors resulting from the inoculation of either H460 or DMS 114 cells of up to 83%. Additionally, cadmium reduced the rate of tumor metastasis to the lung by up to 58%. Cadmium treatment had no effects on either Bcl-2 or Bax protein expression in tumor xenografts, indicating that apoptotic pathways probably do not contribute to this anti-neoplastic effect. These studies show cadmium can effectively reduce growth and progression of human lung carcinoma xenografts in a fashion that is probably independent of apoptosis.

Activation of c-Jun NH2-terminal kinase (JNK/SAPK) in LLC-PK1 cells by cadmium

The level of phosphorylated c-Jun NH2-terminal kinase (JNK) in LLC-PK1 cells treated with CdCl2 increased after 30 min and remained elevated even at 8 hr. And the activity of JNK assayed using glutathione S-transferase-c-Jun as substrate increased dose-dependently. Consistent with the JNK activation, marked increases in the levels of c-Jun and c-Jun phosphorylated on Ser63 and Ser73 were observed in cells treated with CdCl2. The pretreatment with an intracellular Ca2+ chelator, 1, 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA/AM), abolished cadmium-induced JNK phosphorylation. However, pretreatment with a cell permeable chelator of heavy metals, N,N,N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), did not. The present results showed that cadmium induces persistent activation of JNK pathway in a renal epithelial cell line, and that intracellular Ca2+ is necessary for the activation.

Antimony trichloride induces DNA damage and apoptosis in mammalian cells

Antimony compounds are widely used in various manufacturing and semiconducting industries. Previously, it has been shown that antimony trichloride (SbCl3) elevates sister chromatid exchange (SCE) rates in V79 cells after a 28-h incubation. However, only limited data on its genotoxic effects are available so far. The present results demonstrate that a 4-h exposure to > 50 microM SbCl3 could induce micronuclei (MN) formation in cultured Chinese hamster ovary (CHO-K1) cells, human bronchial epithelial (BES-6) cells and human fibroblasts (HF). The order of sensitivity to SbCl3 determined by Sulforhodamine B (SRB)-staining survival assay is HF > BES-6 cells > CHO-K1 cells, with LD50 values in these cells being approximately 40, 80 and 180 microM, respectively. Apoptosis and DNA fragmentation was not found in cells immediately following 4-h SbCl3 treatment. However, DNA fragmentation was detected in CHO-K1 cells after 4-h SbCl3 treatment and a 16 h or more post incubation in fresh medium by 1.5% agarose gel electrophoresis. The delayed apoptosis was also observed under microscopic examination in HF, BES-6 and CHO-K1 cells after similar treatment protocol. In addition, an increase in calcium accumulation appeared in CHO-K1 cells and HF immediately after a 4-h SbCl3 treatment, or after a 24-h post incubation in fresh medium. The present results provide important genotoxic and cytotoxic information characterizing the cellular changes induced by short-term SbCl3 exposure in rodent and human cells.

Exposure of cultured human proximal tubular cells to cadmium, mercury, zinc and bismuth: toxicity and metallothionein induction

The kidney, in particular the proximal convoluted tubule, is a major target site for the toxic effects of various metals. However, little is known about the early effects of these metals after acute exposure in man. In the present study we have evaluated the toxicity of several inorganic metal compounds (CdCl2, HgCl2, ZnCl2, and Bi(NO3)3) and the induction of metallothionein by these compounds in cultured human proximal tubular (HPT) cells for up to 4 days. The results showed that bismuth was not toxic even at the highest dose (100 microM) used, while zinc, cadmium and mercury exhibited varying degrees of toxicity, zinc being the least toxic and mercury the most potent. A significant degree of interindividual variation between the different isolates used in these experiments was also observed. All metals used in the present study induced MT, as revealed by immunocytochemistry. All metals showed maximal induction between 1 and 3 days after treatment. Although a certain amount of constitutive MT was present in the cultures, the intensity of the staining varied with time in culture and between the different isolates studied. No correlation could be made between the intensity of the staining in control cultures (indicating total amount of constitutive MT) and the susceptibility of a given isolate to metal toxicity. Furthermore, no correlation could be made between metal-induced MT and the susceptibility of a given isolate to that particular metal.

Cadmium induces apoptosis differentially on immune system cell lines

We investigate the role of cadmium-induced apoptosis in the immune system, studying the apoptotic effect of Cd2+ in three human cell lines, the T-cell line CCRF-CEM, the B-cell line Raji and the lymphoblastoid cell line Molt-3. Cd2+ was found to be dose-dependently toxic for these cell lines, after 18 h incubation. The 50% lethal dose (LD50) for CCRF-CEM was 25 +/- 20 microM, for Molt-3 was 22.5 +/- 2.4 microM, and for Raji was 13.5 +/- 2.2 microM. DNA electrophoresis and quantitation of apoptosis after 18 h incubation with different Cd2+ concentrations was carried out. In CCRF-CEM cells, apoptosis was detected at 10 microM, reaching a maximum at 30 microM. In Molt-3, apoptosis was detected at 10 microM, increased thereafter and a plateau effect was observed from 30 to 50 microM Cd2+. In Raji, apoptosis was detected at 5 microM, while a plateau effect was observed from 20 to 30 microM Cd2+. The above results indicated that Raji cells were more sensitive to cadmium compared to both CCRF-CEM and Molt-3 cells, suggesting a differential Cd2+-induced apoptotic effect, which may disturb the immune system normal growth and development.

Low-level methylmercury exposure causes human T-cells to undergo apoptosis: evidence of mitochondrial dysfunction

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are immunotoxic to the human immune system. The major focus of our study is to demonstrate that methylmercuric chloride (MeHgCl) kills human lymphocytes by inducing apoptosis. T-cells exposed to 0.6-5 microM MeHgCl for 24 h were analyzed by flow cytometry. Methylmercury-treated cells exhibited increased Hoechst 33258 fluorescence while maintaining their ability to exclude the vital stain 7-aminoactinomycin. Furthermore, T-cells exposed to methylmercury exhibited changes in light scatter patterns that included decreased forward light scatter and increased side light scatter. The light scatter and fluorescent changes were consistent with morphological alterations displayed by cells during apoptosis. Cell death was further evaluated by assessing annexin V binding to the plasma membrane. Methylmercury-treated cells exhibited increased annexin V binding indicative of phosphatidylserine translocation to the outer leaflet of the plasma membrane. Using the fluorescent probe DiOC6(3), we noted that methylmercury exposure resulted in a decrease in mitochondrial transmembrane potential (Psim). Since a low Psim is associated with altered mitochondrial function, we also determined if exposure to methylmercury potentiated reactive oxygen species (ROS) generation. We noted that treated cells generated ROS, as evidenced by oxidation of hydroethidine and the generation of the fluorescent product, ethidium. Finally, we evaluated the effect of methylmercury on T-cell GSH content utilizing the fluorescent probe monochlorobimane; in the presence of MeHgCl, there is a marked loss in reduced cell thiols. The results of the study indicate that a key event in the induction of T-cell apoptosis by mercuric compounds is depletion in the thiol reserve which predisposes cells to ROS damage and at the same time activates death signaling pathways.

Cadmium-induced apoptosis in mouse liver

Apoptosis is a process of active cell death, distinct from necrosis and characterized by specific morphological and biochemical features. Although the acute hepatotoxic effects of cadmium (Cd) are well described, little is known about the occurrence of apoptosis in Cd toxicity. Therefore, mice were injected with 5-60 mumol/kg i.p. of Cd and their livers were removed 1.5-48 h later and examined by light microscopy. Cd induced both a time- and dose-dependent increase in apoptotic index, severity of necrosis, and mitotic index. Apoptotic index peaked at 9-14 h after Cd administration and then decreased. The time course of apoptotic DNA fragmentation index, monitored by quantification of oligonucleosomal DNA fragments, correlated with the results obtained by histopathological analysis and a commercial in situ apoptotic DNA detection kit. Liver necrosis, as demonstrated by histology and serum alanine aminotransferase and sorbitol dehydrogenase assays, was most severe 14-48 h after Cd injection. Apoptosis was decreasing by 24 h while necrosis persisted. Replacement of liver tissue by blood lakes (peliosis hepatis) was observed after 14 h. The mitotic index increased gradually with time, indicating compensatory liver cell regeneration. There was a progressive increase in the severity of necrosis, apoptotic index, and mitotic index with increasing dose of Cd. These data demonstrate that apoptosis is a major mode of elimination of critically damaged cells in acute Cd hepatotoxicity in the mouse, and it precedes necrosis.

Effect of calmodulin-inhibitors and verapamil on the nephrotoxicity of cadmium in rat

Recent reports indicate that calmodulin inhibitors (CIs) can modify cadmium (Cd) toxicity in rodents. Pretreatment with CIs prevents Cd-induced testicular damage in mice and reduces the severity of such damage in rats. On the other hand it has been suggested that the cellular transport of Cd can be partly inhibited by the calcium-channel inhibitor, verapamil. The aim of this study was to determine whether these inhibitors can prevent the toxic effects of Cd on the kidney which is the critical organ. For that purpose, we have examined the effects of two CIs (trifluoperazine and chlorpromazine) and of verapamil on the development of tubular damage in female Sprague-Dawley rats. The animals were injected subcutaneously 5 days a week for 8 weeks with cadmium chloride (1 mg Cd/kg), alone or in association with trifluoperazine (20 mg/kg), chlorpromazine (15 mg/kg) or verapamil (2 x 5 mg/kg). The development of renal dysfunction was followed by measuring the urinary excretion of the low molecular weight protein Clara cell protein (CC16). In Cd-treated rats, the urinary excretion of CC16 started to increase from week 6 to reach at the end of experiment values more than 100-times above normal. CIs or verapamil did not influence the rise of urinary CC16 induced by Cd. The three inhibitors, by contrast, enhanced the accumulation of Cd in the liver and, at the exception of chlorpromazine, in the kidneys of Cd-treated rats. Although interfering with the metabolism of Cd, CIs and verapamil do not prevent renal damage in rats chronically exposed to this heavy metal.

Metal ion-induced toxic histamine release from human basophils and mast cells

Recent data suggest that distinct metal ions can be released from dental alloys or other biomaterials, and may cause toxic effects on various cells. In this study, the effects of 14 metal ions on histamine release from human blood basophils (n = 4), isolated tissue mast cells (lung n = 8, uterus n = 2, skin n = 1, gingiva n = 1), the basophil cell line KU-812, and the mast cell line HMC-1 were analyzed. Of the 14 metal ions, Ag+ (0.33 mM) and Hg2+ (0.33 mM) were found to induce release of histamine in blood basophils, KU-812, mast cells, and HMC-1. The effects of Ag+ and Hg2+ were dose dependent and were observed within 60 min of incubation. In primary mast cells and basophils, AU3+ (0.33 mM) also induced histamine release, whereas no effects of Au3+ on HMC-1 or KU-812 cells were seen. The other metal ions showed no effects on primary or immortal cells within 60 min. However, Pt4+ (0.33 mM) induced histamine liberation in HMC-1 and lung mast cells after 12 h. The Ag+- and Hg2+-induced rapid release of histamine from HMC-1 was associated with ultrastructural signs of necrosis, but not apoptosis. In contrast, prolonged exposure to Pt4+ (0.33 mM, 14 h) induced apoptotic cell death in HMC-1 cells, as assessed by electron microscopy and DNA analysis. Together, certain metal ions induce distinct cytopathogenic effects in mast cells and basophils. Whereas Ag+, Hg2+, and Au3+ cause direct toxicity, Pt4 causes cell death through induction of apoptosis. Whether such effects contribute to local adverse reactions to metal-containing biomaterials in vivo remains to be determined.